This invention provides improvements to various communication devices described in U.S. Pat. No. 6,980,714, dated Dec. 27, 2005, to K. Peter Lo and Norris E. Lewis, entitled “Fiber Optic Rotary Joint and Associated Reflector Assembly”. The communication devices described in the '714 patent are capable of transmitting data and/or power (hereinafter sometimes jointly and severally referred to as “communication”) across a rotary interface, such as between a rotor and a stator.
For example, computed tomography (CT) scanners require data transmission across a rotary interface. In order to enable such data transmission, slip rings are commonly employed. A slip ring has a rotating element that rotates with the rotor, and a stationary element affixed to the stator. Slip rings were originally designed to support electrical communication between the rotor and stator. However, as data rates increased, electrical transmission of the data became impractical. Optical rotary joints were then developed to support higher data transmission rates across the rotary interface. Optical communication is capable of transmitting data at much higher rates than prior electrical communication techniques.
Previous techniques in optical communication across a rotary interface have included the use of waveguides (see, e.g., U.S. Pat. No. 6,453,088, dated Sep. 17, 2002, to Norris E. Lewis, Anthony L. Bowman and Robert T. Rogers, entitled “Segmented Waveguide for Large Diameter Fiber Optic Rotary Joint”; U.S. Pat. No. 6,104,849, dated Aug. 15, 2000, to Norris E. Lewis, Anthony L. Bowman, Robert T. Rogers and Michael P. Duncan, entitled “Fiber Optic Rotary Joint”; and U.S. Pat. No. 5,991,478, dated Nov. 23, 1999, to Norris E. Lewis, Anthony L. Bowman, Robert T. Rogers and Michael P. Duncan, entitled “Fiber Optic Rotary Joint”), optical fibers (see, e.g., U.S. Pat. No. 6,650,843, dated Nov. 18, 2003, to Georg Lohr, Markus Stark and Hans Poisel, entitled “Device for the Optical Transmission of Signals”), and free space propagation (see, e.g., Japanese Pat. Pub. No. 09-308625, dated Feb. 12, 1997, to Suzuki Tatsuro, Teimoshii Aari Fuotsukusu and Tomu Haatofuoodo, entitled “Optical Transmission System”). The aggregate disclosures of these references are hereby incorporated by reference with respect to the structure and operation of such prior art optical rotary joints.
In CT scanner applications, in which the axis of rotor rotation is sometimes physically occupied by a patient, off-axis rotary joints are generally employed to transmit signals between the rotor and stator. Such off-axis rotary joints generally include one or more light sources for emitting the optical signals, and arcuate reflectors having channel-shaped transverse cross-sections that receive such transmitted signals and direct such received signals to respective light receivers. The optical sources are spaced circumferentially about one of the rotor and stator, while the reflectors and receivers are spaced circumferentially about the other of the rotor and stator. The optical sources may include one or more common light sources, the optical signals from which are directed, as by optical fibers, to the periphery of the associated one of the rotor and stator, or may be separate emitting elements mounted about such periphery. For example, the optical sources may be disposed circumferentially about the rotor, while the multiple reflectors and receivers may be disposed circumferentially about the stator, thereby supporting optical communications from the rotor to the stator. In most cases, the path of optical data transmission across the rotary joint (i.e., between the rotor and stator) is in a radial direction with respect to the rotor axis. In other words, if light is transmitted from the rotor to the stator, the light is seen as coming from the rotor axis, for example, regardless of the physical location of the light source(s).
In operation, each of the light sources may possibly transmit the same optical signal. These signals may be transmitted across the rotary interface, and may be received by one or more of the reflectors and be directed to the associated receivers, depending upon the angular position of the rotor relative to the stator. In other embodiments, different optical signals may be transmitted from different light sources, or may be multiplexed if coming from the same source.
While generally effective for permitting optical communication between a rotor and a stator, conventional off-axis rotary joints that employ such arcuate reflectors with channel-shaped cross-sections suffer from several shortcomings, especially at higher data transmission rates. These problems include: (a) the broadening of superimposed pulse widths due to different-length light transmission paths, and (b) that a greater number of light sources must be used when transmitting signals into the entrance end of an optical fiber than when such signals are incident directly upon a photodetector, as discussed infra.
For example, in conventional off-axis rotary joints, the optical signals may travel along different-length paths between the various sources and the respective receivers, thereby introducing time delays in the various received optical signals, when superimposed. A particular receiver might receive signals from two circumferentially-adjacent optical sources. If the same optical signal is simultaneously emitted by the two adjacent sources, but such signals travel different distances to reach the receiver, the signals will be received at different times. Accordingly, the two signals will be out-of-phase, and the pulse width of the superimposed signals, as seen by the receiver, will be effectively broadened. To support communication at the desired high data rates, conventional offaxis rotary joints have been specifically designed to have less spacing between the optical sources and the receivers so as to minimize the path lengths of signal travel. Even so, it is difficult to support error-free data transmission at a data rate above 1.25 Gbit/sec, where the signals travel along different-length paths.
The aforesaid '714 patent discloses an optical rotary joint and an associated reflector assembly to provide optical communication between a rotor and a stator. By designing the optical rotary joint such that the optical signals travel along paths of equal path, the pulse width of the superimposed optical signals, as seen by the receiver, will not be increased.
To effect this, the '714 patent contemplates that the rotary joint include a reflector assembly having a concave elliptical reflecting surface, and possibly a hyperbolic reflecting surface as well. Both shapes are conic-section curves in a Cartesian plane (i.e., defined by the x-y axes) defined by the general equation:Ax2+Bxy+Cy2+Dx+Ey+F=0where A, B, C, D, E and F are constants. For an ellipse, B2<4AC; for a hyperbola, B2>4AC. For an ellipse, the sum of the distances from any point on the curve to the two focal points (F1, F2) is a constant. If the reflecting surface is configured as a portion of an ellipse, light issuing from one focal point will be reflected by such elliptical reflecting surface toward the other focal point. However, the total length of the path of light traveling from one focal point to the other will be a constant, regardless of the specific location of the point on the elliptical reflecting surface on which the emitted light is incident. Conversely, for a hyperbolic reflecting surface, the difference between the distances from any point on the curved reflective surface to the two fixed focal points is a constant.
The '714 patent discloses several different optical configurations. In some of these, the received signals are focused directly on a photodiode. In other configurations, the reflected signals are focused on the entrance end of an optical fiber that communicates with a photodiode at a remote location. In still other configurations, a focusing lens is positioned at the entrance end to direct the received signals into the optical fiber.
However, the acceptance angle of an optical fiber is more limited than that for a photodiode. The main reason for this is that an optical fiber has a more limited numerical aperture (NA), than does a photodiode. The more-limited NA of an optical fiber restricts the approach angle at which light can be directed and guided into the fiber. This, in turn, limits the design of the reflective surfaces by means of which light may be directed toward the entrance end of the receiving fiber. This limitation requires, as a practical matter, that a greater number of light sources be used when the transmitted signals are to be directed initially into an optical fiber, than when such signals are incident directly on a photodetector.
Referring now to the drawings, FIGS. 1 and 2 of the present application correspond substantively to FIGS. 1 and 2 of the '714 patent, except for the differences in the reference numerals. Thus, these figures disclose a prior art optical rotary joint, generally indicated at 20, in which the various light sources 21 are mounted on the rotor 22. The optical beams are directed radially outwardly, as if they were coming from focal point F1 at the rotor's axis of rotation. The beams are incident on the elliptical reflecting surface 23 of reflector 24, and are reflected backwardly toward the conjugate focal point F2. However, such reflected beams are incident on, and are re-reflected forwardly by, hyperbolic reflecting surface 25 positioned between the elliptical reflecting surface and the back focal point B, and such forwardly-reflected beams are focused on receiver 26 which is located at the conjugate focal point C. The back focal point B of hyperbolic reflecting surface 25 is coincident with conjugate focal point F2 of elliptical reflecting surface 23.
The '714 patent discloses an optical rotary joint that allows the transmission of high bandwidth optical signals from the rotor to the stator, and vice versa. In the scenario in which optical signals are transmitted from the rotor to the stator, a number of light sources are spaced evenly around the periphery of the rotor. The number of light sources required for continuous transmission of data across the rotary interface depends on the acceptance angle, θ, of the elliptical reflector. The acceptance angle δ is defined as the angle of the elliptical reflective surface, measured from the center of the rotor within which the light rays from the source can be directed and guided into the receiver. The acceptance angle θ is a function of the optical path length and the acceptance angle φ of the receiving fiber or photodetector (as appropriate), where:φ=2×sin−1 (NA)
To insure that optical communications can be continuously transmitted, at least one optical source has to be within the acceptance angle of the elliptical reflector at all relative angular positions of the rotor and stator. For example, if the receiving fiber has an NA of 0.37, the acceptance angle φ of this configuration is 13.6°, as shown in FIG. 3. Beyond the limits of this acceptance angle, the optical signals simply attenuate in the cladding layer of the fiber, and do not reach the photodetector. To populate the rotor circumference with evenly-spaced light sources such that at least one source is within the acceptance angle of the receiving fiber at all such relative angular positions, at least twenty-seven sources are required (i.e., 360°/13.6°=26.47 27 sources).
On the other hand, if a photodetector is used as the receiver, and if the photodetector has an NA of 0.74 such that its acceptance angle φ is widened to 32°, then only twelve circumferentially-spaced light sources are needed to populate the rotor so as to assure continuous communication (i.e., 360°/32°=11.25≈12 sources). Depending on the particular design of the photodiode package, the acceptance angle can be as high as 140° (NA=0.94).
Thus, to reduce the number of sources and to reduce cost and system complexity, it would be more advantageous to have the received signals are incident directly on a photodetector, rather than be first directed into the entrance end of an optical fiber for transmission therealong to a remotely-located photodetector. Moreover, a shorter path length to the photodetector is also desirable to reduce angular tolerance issues in a production environment.
Referring now to FIG. 4 of the present application, which substantively corresponds to FIG. 4 of the '714 patent except for the differences in the reference numerals, the '714 patent also discloses an embodiment, generally indicated at 28, in which a single elliptical reflector 29 is used (i.e., without a cooperative hyperbolic reflector), and the conjugate focal point F2 lies radially outside of the rotor. In this arrangement, reflector 29 has an elliptical reflective surface 30 operatively arranged to focus the beams emanating radially outwardly from sources 31 into the entrance end of an optical fiber 32 that communicates with a remotely-located photodetector (not shown). The entrance end of the optical fiber is coincident with the conjugate focal point F2 of the elliptical reflector.
If the reflected light is directed into optical fiber 32, the limited NA of the receiving fiber again requires that a large number of light sources to be spaced equally about the circumference of the rotor. For example, when the NA of the fiber is 0.37, using geometrical analysis, the acceptance angle of the reflector, θ, is 9.7°, with an optical path length of 210 mm from the edge of the rotor to the receiver. To populate the rotor such that continuous transmission of data from the rotor to the stator is assured, at least thirty-eight light sources are required (i.e., 360°/9.7°=37.11≈38 sources).
These two examples demonstrate that when the reflected light beams are incident directly on a receiver having a larger NA, such as a photodetector as opposed to the entrance end of an optical fiber leading to the receiver, the number of light sources, and hence the cost and complexity of the system, may be reduced.
While the fiber optic rotary joint in the '714 patent can enable high data trans-mission across a rotary interface, it would be desirable to provide improved versions of such constant-path-length fiber optic rotary joints that are capable of transmitting optical signals at rates greater than about 1.25 Gbit/s, that have lower insertion losses, that are more compatible with the use of optical fibers leading to remote receivers, that use smaller numbers of light sources, and that have minimum optical path lengths